Synonyms

Defective visual localization; Ocular dysmetria; Visuomotor ataxia

Short Description or Definition

Optic ataxia is a deficit of visually guided hand movements toward a normally perceived object in the peripheral visual field(s).

Epidemiology

Unknown, rare.

History

Deficits in reaching movements have been described since the late nineteenth century (Pisella et al. 2008). In 1909, Rezsö Bálint (1909) used the term “optische ataxie” to describe the difficulties one of his patients had in making visually guided hand movements with his right hand. Reaching with his left hand was accurate. Reaching for parts of his body with his right hand with his eyes closed was accurate. Thus, the problem was neither visual nor motor, but rather visuomotor. The term “optic ataxia” was apt. The patient presented with additional symptoms of a significant restriction of visual attention and unilateral neglect. At autopsy, the patient was found to have bilateral occipitoparietal lesions.

Holmes (1918) described a related syndrome of “visual disorientation” in soldiers with bilateral parietal lesions. These patients had significant difficulty in maintaining visual fixation, and their gaze wandered when searching for objects in peripheral vision.

Optic ataxia is often encountered in the context of Bálint’s syndrome, which is now generally considered to consist of a triad of symptoms including (1) a deficit of visually guided hand movements (optic ataxia), (2) difficulties in visual fixation and scanning (“optic apraxia”), and (3) an extreme restriction of visual attention (similar to “simultanagnosia”). Optic ataxia can be a persistent and residual deficit in patients recovering from Bálint’s syndrome. It can also occur in isolation, but it was not until 1967 when optic ataxia was identified as a specific entity separate from Bálint’s syndrome (Garcin et al. 1967).

Clinical Presentation

Bálint’s patient described his right hand as clumsy and reported often lighting a cigarette in its middle instead of its end. A patient with optic ataxia might reach for food on someone else’s plate rather than his own. On neurological exam, the patient may appear to have cerebellar ataxia, with overshooting and undershooting the target. The difference is that with optic ataxia, patients can reach accurately if they have proprioceptive cues. They also lack intention tremor and dysdiadochokinesia.

There may also be other impairments in optic ataxia, such as difficulty shaping the hand correctly for grasping, inability to correct ongoing movements, and failure to recognize obstacles in the path to the object. It is also possible to see impaired ability to touch an object with a toe under visual guidance alone.

Neuropsychology of Optic Ataxia

Patients with optic ataxia have a deficit in reaching out to objects in their peripheral visual field, despite normal recognition and naming of the object and in the absence of primary visual or motor deficits. Reaching is normal when performed with central fixation of the object. With bilateral lesions, as in Bálint’s syndrome, optic ataxia is apparent in both visual fields, and there may or may not be an inferior quadrant visual field defect. Isolated optic ataxia following a unilateral lesion results in contralateral symptoms (“field effect”), with greater relative errors performed by the contralesional hand (“hand effect”). Proprioception is largely spared as patients can, with eyes closed, point to their body parts touched by the examiner and point to named body parts (e.g., their nose) (Pisella et al. 2008). Isolated optic ataxia is associated with lesions located around the intraparietal sulcus and superior parietal lobe or from disconnections from the visual and visuomotor areas to the frontal lobe (Devinsky and D’Esposito 2004). Bálint’s syndrome is a result of bilateral damage to the occipitoparietal region. This is usually due to infarctions in the border zone (watershed) between the territories of the anterior and posterior cerebral arteries from sudden and severe hypotension (Damasio et al. 2000).

The neuropsychology of optic ataxia is incompletely understood. Andersen et al. (2014) provide a detailed systematic review of ongoing research. Optic ataxia is now understood to be a high-order deficit, at an integrative sensorimotor level, that can occur with lesions in the posterior parietal cortex (PPC) associated with a variety of conditions, ranging from neurodegenerative diseases such as posterior cortical atrophy to traumatic brain injury, brain tumors, and infections. Pure optic ataxia is more rare than Bálint’s syndrome because of the more localized and discrete lesions that produce optic ataxia alone. It has been found that optic ataxia in isolation can be caused by damage to a specific region within the PPC called the parietal reach region.

As described by Andersen et al. (2014), there are currently three frameworks proposed for optic ataxia: disruption of (1) visuomotor processing, (2) visual orientation, or (3) online visuomotor control. It is difficult to distinguish between these models with human case reports alone, so research is ongoing with nonhuman primate and animal subjects. Each framework is conceptualized as consisting of multiple functional modules. Bálint’s syndrome and optic ataxia are believed to be caused by disruption of multiple functional modules within the PPC. Research into the framework for optic ataxia has the added benefit of elucidating the functional organization of the PPC for action planning.

Outcomes and Treatment

Functional outcome in cases of isolated optic ataxia is usually good because the lesion is not extensive and reaching behavior under conditions of central fixation is normal and visuospatial functions are spared. Patients spontaneously adapt to the deficit by moving central fixation to the target in their peripheral visual field. Functional prognosis is poor in full Bálint’s syndrome due to the extent of the bilateral lesions and the presence of the other symptoms of the syndrome (Pisella et al. 2008).

Cross-References